Observation of Sawtooth-like Generation of Plasma Current During Whistler Activities

We have observed semi-periodic increases in plasma current in a sawtooth-like bursts that are correlated with Whistler waves and with x-ray activities. These activities occur in tokamak plasma in the Madison Symmetric Torus only when plasma density is low, ne < 0.02x10¹³ cm^-3. The period of these current bursts is similar to the period of high frequency magnetic fluctuations of Whistler waves, and the period of radially emitted x-ray flux. These high frequency magnetic fluctuations are consistent with anomalous doppler resonance estimate. The period of these activities increases as the plasma density increases. The plasma current saw teeth are strongest when the plasma current is ramping down and density is low consistent with changes in the toroidal electric field. The increases in plasma current at the sawtooth is as large as 2% of the background plasma current. The toroidal electric field shows a decrease in the form of a narrow pulse. The plasma current increases in a fraction of ms followed by 1 to 2 ms ramp down. These activities occur in discharges with I_P = 50-70 kA, B_t = 1310 gauss, and q(a) = 2.2-1.6. Further work is needed to characterize these saw teeth as a result of dynamo activities and/or driven by runaway electrons. Data and analyses will be presented.

* Work supported by U.S. DOE funding for WiPPL DE-SC0018266, and NSF award PHY 1828159.